The present exemplary embodiment relates to electronic ballasts. It finds particular application in conjunction with the high intensity discharge lamps (HID), and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other electronically ballasted lamps such as fluorescent lamps and the like.
A ballast is an electrical device which is used to provide power to a load, such as an electrical discharge lamp, and to regulate its current. The ballast provides high voltage to start a lamp, causing the gas to ionize which begins the process of arc formation. Once the arc is established, the ballast allows the lamp to continue to operate by providing proper controlled current flow to the lamp.
An important indicator of lamp current quality for a high intensity discharge lamp is the current crest factor (CCF) of the lamp current. A low CCF, i.e. one that approaches unity, is preferred because a high CCF can cause the deterioration of the lamp which would subsequently reduce the life of the lamp. The crest factor of a waveform is defined as the peak divided by the RMS value. Ideally, square waves have a crest factor of unity since the RMS and the peak amplitude are equal. Typically, the ballasts which provide crest factor close to unity allow for maximum lamp life, for example 40,000 service hours. The crest factor is sensitive to the existence of sharp peaks in the waveform which are of a short time duration since the RMS value is proportional to the amount of energy in the signal. E.g., the short terms peaks do not contain much energy.
Typically, low frequency, square wave ballasts include a three stage power conversion process. Initially, at stage 1, the AC power line voltage is rectified and filtered. At the intermediate stage 2, the DC voltage is converted to DC current. At stage 3, the DC current is converted to an AC current. A commutator or inverter, which inverts the DC into the AC, periodically unloads the intermediate stage during the commutator's transition intervals. This causes the output voltage of the intermediate stage, which supplies the power to the lamp, to abruptly change to a higher level for a brief period of time. For example, the voltage abruptly jumps from 90 Volts to 120 Volts during the commutator's transition intervals. Often this causes the current crest factor to exceed the maximum set value of 1.5, during the steady-state operation, or 1.8 during warm-up.
One approach to reduce the lamp current crest factor is to use unidirectional snubber circuits.
The present application contemplates new methods and apparatuses that overcome above referenced problems and others.